The invention relates to a method of operating an internal-combustion engine.
In a conventional turbo-engine, that is to say a supercharged engine, which has a geometrical compression ratio which is acceptable from the point of view of fuel consumption, it is always necessary at high load to set ignition relatively late so as to avoid knocking. This means that the effective expansion ratio decreases and that less work can be extracted from the combusted gas. A high exhaust-gas temperature at high engine speed also results.
Moreover, if the turbo-engine has a relatively small turbine in order to improve torque and response at low engine speeds, a further increased exhaust-gas temperature is obtained as a result of reduced mass flow through the turbine. The usual method of lowering the exhaust-gas temperature is to run the engine with an extremely rich mixture (with an air/fuel ratio of the order of 11:1) at high engine speeds.
This results in the temperature increase during combustion being reduced, which results in cooler exhaust gases. At the same time, however, there is a great increase in emissions of uncombusted hydrocarbons and of carbon monoxide. Conversion in the catalyst is insufficient as a result of a lack of oxygen in the exhaust gases.
A further disadvantage is that the fuel consumption increases by roughly 30%.
An advantage, however, is that the quantity of nitrogen oxides decreases in comparison with the case of a stoichiometric mixture.
A transition to stoichiometric operation would improve the situation as far as emissions are concerned but would on the other hand cause a decrease in power of the order of 25% as a result of the need to set ignition earlier in order to limit the increase in the exhaust-gas temperature which otherwise occurs. However, ignition being set earlier results in a greatly increased maximum pressure in the cylinder with the attendant risk of knocking. This in turn leads to a significant reduction of the charging pressure being necessary. Overall, this would result in great problems for turbo-engines.
The aim of the invention is to improve the characteristics of a turbo-engine, in particular at high load. Another aim is to bring about a cost-effective solution.
The aim of the invention is achieved on the one hand by using an operating method in which at high load the engine is run with a lean mixture during supercharging, the air/flow ratio being at least 19:1.
By making it possible to operate the engine, with a lean mixture during supercharging and at high load, it is possible to reduce to a significant extent the previously common high emissions of uncombusted hydrocarbons and of carbon monoxide during running of a turbo-engine at high load. Moreover, a lower fuel consumption is achieved.
The lean mixture is brought about by increasing the charging pressure while maintaining the fuel quantity. Operation and engine are especially advantageous when the technique using a divided exhaust-gas period is used in order to obtain efficient turbine operation and efficient cylinder emptying during the exhaust stroke.
Further features and advantages of solutions according to the invention emerge from the description and the other patent claims.
The invention will be described in greater detail below with reference to exemplary embodiments shown in the appended drawing.